Semiconductor devices have many functions in a plurality of industrial fields, including, but not limited to, fabrication of electronic devices, such as transistors, and photovoltaic cells, such as solar cells. Individual photovoltaic cells are e.g. used for powering small devices such as electronic calculators. Photovoltaic arrays are used for instance in remote area power systems, earth-orbiting satellites and space probes, remote radiotelephones and water pumping applications.
The principle of operation of a solar cell containing a p-n junction can be roughly described as follows. The solar cell absorbs light and generates electron/hole charge pairs due to absorbed light energy. The electrons move toward the n-layer side of the junction, and the holes move toward the p-layer side due to drift caused by the junction electric field and diffusion.
For solar cell production, so called bulk technologies or thin-film technologies may be applied, the former utilizing bulk semiconductor wafers, the latter resulting in thin-film solar cells. The most prevalent bulk material of solar cells is crystalline silicon, for instance mono-crystalline silicon (c-Si), or multi-crystalline silicon (mc-Si).
A typical solar cell production process based on mc-Si may include for instance some of the following steps: Removal of saw-damage from mc-Si wafers by etching; emitter formation by heat treatment in POCl3 ambient; removal of resulting phosphorus silicate glass (PSG); front side and/or back side passivation; deposition of a front side anti-reflection coating, e.g. by Plasma Enhanced Chemical Vapor Deposition (PECVD); screen printing of rear side metallization and front contact grid lines; and annealing for rear side metal interdiffusion and firing the front contact grid lines.
Multi-crystalline silicon wafers may contain a high density of recombination centers, for instance lattice defects, e.g. impurities, such as iron (Fe) or nickel (Ni). This results in undesirable recombination of charge carriers, thereby affecting the electrical properties of the solar cell. To improve the properties of solar cells based on mc-Si, a so-called gettering of impurities, and thereby inactivation of impurities, may be performed, which typically includes a phosphorus diffusion at high temperatures. Phosphorus is able to retain, for instance, Fe and other metallic impurities in the emitter and/or surface of the solar cell, reducing the recombination due to these impurities. During phosphorus diffusion, a phosphorus silicate glass is formed, which has to be removed for the subsequent solar cell production process steps.